Dynamic Voltage and Frequency Management

ABSTRACT

In one embodiment, an integrated circuit includes a self calibration unit configured to iterate a test on a logic circuit in the integrated circuit at respectively lower supply voltage magnitudes until the test fails. A lowest supply voltage magnitude at which the test passes is used to generate a requested supply voltage magnitude for the integrated circuit. In an embodiment, an integrated circuit includes a series connection of logic gates physically distributed over an area of the integrated circuit, and a measurement unit configured to launch a logical transition into the series and detect a corresponding transition at the output of the series. The amount of time between the launch and the detection is used to request a supply voltage magnitude for the integrated circuit.

This application is a continuation of U.S. patent application Ser. No.13/032,052, filed Feb. 22, 2011, which is a divisional of U.S. patentapplication Ser. No. 12/361,405, filed Jan. 28, 2009, now U.S. Pat. No.7,915,910.

BACKGROUND

1. Field of the Invention

This invention is related to integrated circuits and, more particularly,to dynamic voltage and frequency management in an integrated circuit.

2. Description of the Related Art

As the number of transistors included on a single integrated circuit“chip” has increased and as the operating frequency of the integratedcircuits has increased, the management of power consumed by anintegrated circuit has continued to increase in importance. If powerconsumption is not managed, meeting the thermal requirements of theintegrated circuit (e.g. providing components required to adequatelycool the integrated circuit during operation to remain within thermallimits of the integrated circuit) may be overly costly or eveninfeasible. Additionally, in some applications such as battery powereddevices, managing power consumption in an integrated circuit may be keyto providing acceptable battery life.

Power consumption in an integrated circuit is related to the supplyvoltage provided to the integrated circuit. For example, many digitallogic circuits represent a binary one and a binary zero as the supplyvoltage and ground voltage, respectively (or vice versa). As digitallogic evaluates during operation, signals frequently transition fullyfrom one voltage to the other. Thus, the power consumed in an integratedcircuit is dependent on the magnitude of the supply voltage relative tothe ground voltage. Reducing the supply voltage generally leads toreduced power consumption, but also impacts the speed at which digitalcircuits operate and thus may cause incorrect operation at a givenoperating frequency (that is, the frequency at which digital logic inthe integrated circuit is clocked) or may reduce performance.

Additionally, as transistor geometries have continued to decrease insize, leakage currents that occur when a transistor is not activelyconducting current have become a larger component of the power consumedin the integrated circuit. The amount of leakage current experienced ina given transistor generally increases linearly as the supply voltageincreases. Additionally, at each new semiconductor fabrication processnode (in which the transistor geometries decrease), the leakage currentincreases more than the active (ON) current. Thus, as more advancedprocess nodes are used, the leakage current becomes a larger and largerissue.

Thus, power consumption in an integrated circuit may be managed bylowering the supply voltage to the integrated circuit, but incorrectoperation may also result if the supply voltage is reduced too far. Thesupply voltage magnitude at which incorrect operation occurs for a givenoperating frequency varies on part-by-part basis for a given integratedcircuit design. For example, variations in the integrated circuitmanufacturing process used to manufacture the integrated circuit and theoperating temperature of the integrated circuit may both impact thesupply voltage magnitude at which incorrect operation occurs.Accordingly, attempts to manage power consumption via the supply voltagehave been limited to supply voltage magnitudes that ensure correctoperation at the given frequency across all acceptable variations in themanufacturing process and all permissible operating temperatures.Typically, the supply voltage for a given frequency is staticallyspecified in the integrated circuit's specification.

SUMMARY

In an embodiment, an integrated circuit comprises a logic circuit, alocal power manager coupled to the logic circuit, and a self calibrationunit. The local power manager is configured to transmit an indication ofa requested supply voltage magnitude to an external power supply. Theself calibration unit is configured to execute a test on the logiccircuit, and to iterate the test at respectively lower requested supplyvoltage magnitudes until the test fails. A lowest requested supplyvoltage magnitude at which the test passes is used to generate therequested supply voltage magnitude for operation of the integratedcircuit.

In an embodiment, a method comprises iterating a test on a logic circuitby a self calibration unit at respectively lower requested supplyvoltage magnitudes for an integrated circuit that includes the logiccircuit and the self calibration unit until the test fails. The methodfurther comprises the self calibration unit determining a lowestrequested supply voltage magnitude at which the test passes. The methodstill further comprises the self calibration unit selecting the lowestrequested supply voltage magnitude to generate the requested supplyvoltage magnitude for operation of the integrated circuit.

In an embodiment, an integrated circuit comprises a plurality of logicgates physically distributed over an area of the integrated circuit thatis occupied by a logic circuit that implements the operation of theintegrated circuit, wherein the plurality of logic gates are connectedin series; and a measurement unit coupled to a first gate in the seriesand a last gate in the series. The measurement unit is configured tolaunch a logical transition into the first gate and to measure timeuntil a corresponding transition is detected from the last gate. Themeasured time is compared to a predetermined time to adjust a supplyvoltage of the integrated circuit. In some embodiments, thepredetermined time may be determined during a self-calibrationprocedure. In some embodiments, the predetermined time may be measuredas a number of clock cycles that it takes for the pulse to travelthrough all the gates in series.

In an embodiment, a method comprises a measurement unit launching alogical transition into a first gate of a series connection of aplurality of gates and that are physically distributed over an area ofan integrated circuit that is occupied by a logic circuit thatimplements the operation of the integrated circuit; and the measurementunit measuring a time until a corresponding transition is detected fromthe last gate, wherein the measured time is compared to a predeterminedtime to adjust a supply voltage of the integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 is a block diagram of one embodiment of an integrated circuit.

FIG. 2 is a flowchart illustrating one embodiment of a test of theintegrated circuit shown in FIG. 1.

FIG. 3 is a flowchart illustrating operation of one embodiment of a selfcalibrate unit shown in FIG. 1.

FIG. 4 is a flowchart illustrating operation of one embodiment of theintegrated circuit shown in FIG. 1 to change a frequency of operation ofthe integrated circuit.

FIG. 5 is a flowchart illustrating performance of self calibration inresponse to various events, for one embodiment.

FIG. 6 is a block diagram of another embodiment of the integratedcircuit.

FIG. 7 is a flowchart illustrating one embodiment of a test of theintegrated circuit shown in FIG. 6.

FIG. 8 is a flowchart illustrating operation of one embodiment of theintegrated circuit to request a supply voltage.

FIG. 9 is a flowchart illustrating operation of one embodiment of theintegrated circuit shown in FIG. 6 to change a frequency of operation ofthe integrated circuit.

FIG. 10 is a flowchart illustrating another embodiment of a test of theintegrated circuit shown in FIG. 6.

FIG. 11 is a flowchart illustrating operation of one embodiment of aspeed/temperature compensation unit shown in FIG. 6.

FIG. 12 is a graphical representation of the number of parts that areoperable at various supply voltages and the test voltages that may beused in one embodiment of testing the integrated circuit.

FIG. 13 is a graphical representation of the number of parts that areoperable at various supply voltages and the test voltages that may beused in another embodiment of testing the integrated circuit.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims. The headings used herein are for organizational purposes onlyand are not meant to be used to limit the scope of the description. Asused throughout this application, the word “may” is used in a permissivesense (i.e., meaning having the potential to), rather than the mandatorysense (i.e., meaning must). Similarly, the words “include”, “including”,and “includes” mean including, but not limited to.

Various units, circuits, or other components may be described as“configured to” perform a task or tasks. In such contexts, “configuredto” is a broad recitation of structure generally meaning “havingcircuitry that” performs the task or tasks during operation. As such,the unit/circuit/component can be configured to perform the task evenwhen the unit/circuit/component is not currently on. In general, thecircuitry that forms the structure corresponding to “configured to” mayinclude hardware circuits and/or memory storing program instructionsexecutable to implement the operation. The memory can include volatilememory such as static or dynamic random access memory and/or nonvolatilememory such as optical or magnetic disk storage, flash memory,programmable read-only memories, etc. Similarly, variousunits/circuits/components may be described as performing a task ortasks, for convenience in the description. Such descriptions should beinterpreted as including the phrase “configured to.” Reciting aunit/circuit/component that is configured to perform one or more tasksis expressly intended not to invoke 35 U.S.C. §112, paragraph sixinterpretation for that unit/circuit/component.

DETAILED DESCRIPTION OF EMBODIMENTS

Turning now to FIG. 1, a block diagram of one embodiment of anintegrated circuit 10 coupled to an external power management unit(PMU)/power supply 12 is shown. In the illustrated embodiment, theintegrated circuit 10 includes a logic circuit 14, a self calibrationunit 16, a local power manager 18 (which may include a self calibrationtable 20), and a frequency/voltage (F/V) table 22. The self calibrationunit 16 and the F/V table 22 are coupled to the local power manager 18,which is coupled to transmit an indication of a requested supply voltagemagnitude (V_(DD) request) to the PMU/power supply 12. The PMU/powersupply 12 is coupled to provide a supply voltage (V_(DD)) of therequested magnitude to the integrated circuit 10. The componentsillustrated within the integrated circuit 10 are integrated onto asingle semiconductor substrate, or chip.

Generally, the self calibration unit 16 comprises control circuitryalong with a test that is to be executed by the logic circuit 14. Thetest may be intended to exercise known “critical” timing paths in thelogic circuit 14. A critical timing path may be a path through thecircuitry that is expected to exhibit the highest delay (as compared toother timing paths) from input transition to corresponding outputtransition, and thus would be a path that limits the operating frequencyat which the logic circuit 14 will operate correctly. The nature of thetest may vary depending on the definition of the logic circuit 14. Forexample, if the logic circuit 14 includes one or more processor cores,the test may comprise a software program that is to be executed by theprocessor core(s), along with an expected result from the program (e.g.a signature). If the logic circuit 14 includes fixed-function circuitry,the test may include input signal values and expected output signalvalues. A combination of signal values and program instructions may beincluded, in various embodiments.

The circuitry in the self calibration unit 16 may be configured toexecute the test on the logic circuit 14 (e.g. supplying theinstructions to the processor core(s) for execution, and/or driving thesignals with the input signal values). The circuitry in the selfcalibration unit may also be configured to check the result against theexpected value. The self calibration unit 16 may be configured toiterate the test, and to communicate with the local power manager 18 torequest lower supply voltage magnitudes for each iteration, until anincorrect result is detected for an iteration. The lowest supply voltagemagnitude for which the correct result of the test is detected may beprovided as the supply voltage magnitude to request (or some margin maybe added to the lowest supply voltage magnitude to arrive at themagnitude to be requested). The self calibration unit 16 may iterate thetest for each possible operating frequency, or may perform the test fora given operating frequency in response to the first time that the givenoperating frequency is actually requested for the integrated circuit 10(e.g. by software).

By using the self calibration unit 20, in some embodiments, a smallermargin may be used since the self calibration occurs with the integratedcircuit 10 installed in the particular device in which it will bedeployed (and thus some factors that are to be accounted for withmargin, such as variation in the power supply 12, the board design, thepackage of the integrated circuit 10, etc.) are relatively fixed.Additionally, rather than testing for the lowest possible supply voltageat manufacturing test, fewer supply voltage magnitudes may be tested atthat time and thus manufacturing test time may be reduced, in someembodiments. Additionally, the self calibration unit 16 may be activatedat any time, thus automatically adjusting for aging effects in theintegrated circuit 10, in some embodiments.

In one embodiment, the local power manager 18 may store the resultingsupply voltage magnitudes provided by the self calibration unit 16 inthe self calibration table 20. The self calibration table 20 may be arandom access memory, clocked storage devices such as registers, or anyother volatile memory. Alternatively, non-volatile memory such asprogrammable read-only memory, flash memory, etc. may be used.Thereafter, if an entry in the self calibration table 20 is detected fora given operating frequency, the supply voltage magnitude recorded inthe entry may be requested by the local power manager 18.

The F/V table 22 may comprise a plurality of entries, each storing arespective operating frequency for the integrated circuit 10 and acorresponding supply voltage magnitude for that frequency. The frequencyof operation may be the frequency for the clock that is supplied to theclocked storage devices in the logic circuit 14. There may be a set offrequencies at which the integrated circuit 10 may operate (andswitching between the frequencies in the set may be supported by theintegrated circuit 10, e.g. to permit power management, thermalmanagement, etc.). The F/V table 22 may be a static table written duringmanufacturing test of the integrated circuit 10 (e.g. prior to packagingthe integrated circuit, such as at wafer test). In other embodiments,the test may be performed at any point prior to selling the integratedcircuit 10 for inclusion in a device, or prior to including theintegrated circuit 10 in such a device. In still other embodiments, theF/V table 22 may be written during a self calibration that may beperformed prior to using a device including the integrated circuit 10for the first time. Thus, the supply voltage magnitude determined foreach frequency in the F/V table 22 may have significant guardbandingassociated with it to ensure correct operation in the event that thepackage's electrical characteristics change the voltage magnitude, toaccount for thermal variation (e.g. the test may be performed at acontrolled temperature, and the operating temperature may be higher orlower than that temperature), to account for aging effects in theintegrated circuit over its expected life, etc.

The local power manager 18 comprises circuitry that is configured torequest a supply voltage magnitude from the external power supply (e.g.the PMU/power supply 12). As mentioned previously, if an entry in theself calibration table 20 is detected for a given operating frequency,the local power manager 18 may request the supply voltage magnituderecorded in that entry. If no entry is found in the self calibrationtable 20, the local power manager 18 may read the F/V table 22 for thegiven operating frequency and may request that supply voltage magnitudefrom the PMU/power supply 12 (V_(DD) request in FIG. 1). The request maybe represented in any desired fashion. For example, the request maycomprise a plurality of bits, with various supply voltage magnitudeswithin a range of supported magnitudes each assigned a differentencoding of the plurality of bits.

The local power manager 18 may also be configured to control the changebetween operating frequencies. For example, the local power manager 18may include a register or other facility that can be written by softwareto select a new operating frequency. The local power manager 18 maydetect the write, and may manage the transition from the currentoperating frequency to the newly requested operating frequency. Thetransition may include changing the requested supply voltage, changingthe operation of clocking circuitry (e.g. relocking a phase locked loop(PLL) that generate the clocks on the integrated circuit 10, etc.), etc.Thus, the details of the transition may be abstracted from software,which may simply request the new frequency and continue (e.g. withouteven checking to see if the transition has completed), in oneembodiment.

The F/V table 22 may be written in any desired fashion. For example,each entry in the table may comprise fuses that may be selectively blownto permanently store an indication in the entry of the desired voltagemagnitude (e.g. encoded as a plurality of bits in the entry). Any othernon-volatile storage may be used, in other embodiments. The F/V table 22may comprise a non-volatile memory that may be written via an update tothe firmware of the device that includes the integrated circuit 10.

In some embodiments, the test that is executed by the self calibrationunit 16 may be programmable, and may be updated. Such embodiments maypermit the test to be changed as more data becomes available. Forexample, a path other than the previously-identified critical paths maydominate, or strongly affect, the supply voltage at which the integratedcircuit 10 operates correctly. The test may be updated to include thenewly discovered critical path. Still further, the test may be updatedto include a more pertinent program to execute during the test, in someembodiments.

The logic circuit 14 may generally include the circuitry that implementsthe operation for which the integrated circuit 10 is designed. Forexample, if the design includes one or more processors, the logiccircuit 14 may include the circuitry that implements the processoroperation (e.g. instruction fetch, decode, execution, and result write).The processors may include general purpose processors and/or graphicsprocessors in various embodiments. If the design includes a bridge to aperipheral interface, the logic circuit 14 may include the circuitrythat implements the bridge operation. If the design includes othercommunication features such as packet interfaces, network interfaces,etc., the logic circuit 14 may include circuitry implementing thecorresponding features. The integrated circuit 10 may generally bedesigned to provide any set of operations. Generally, the logic circuit14 may comprise any combination of one or more of the following: memoryarrays, combinatorial logic, state machines, flops, registers, otherclocked storage devices, custom logic circuits, etc.

The PMU/power supply 12 may generally include any circuitry that iscapable of generating a supply voltage of a magnitude indicated by aninput voltage request. The circuitry may include one or more voltageregulators or other power sources, for example. The PMU/power supply 12may also include power management circuitry for the system (thatincludes the integrated circuit 10) as a whole.

While the above discussion has referred to requesting a supply voltagemagnitude, and the PMU/power supply 12 supplying a voltage of therequested magnitude, the discussion is not meant to imply that there isonly one requested/supplied voltage. There may be multiple supplyvoltages requested and supplied at any given point in time. For example,there may be separate supply voltages for combinatorial logic circuitryand for memory circuitry in the logic circuit 14. There may be multiplevoltage domains within the integrated circuit 10 that may be powered upand down separately, and each domain may include a separate request. Thelocal power manager 18 may be powered separate from the logic circuit14. Any set of one or more supply voltages may be requested andsupplied.

The magnitude of the supply voltage has been referred to above as beingrequested, and the supply voltage of the requested magnitude beingsupplied. The magnitude of the supply voltage may be measured withrespect to a reference (e.g. the ground of the integrated circuit 10,sometimes referred to as V_(SS)). For convenience in the descriptionbelow, voltages may be referred to as being greater than or less thanother voltages. Similarly, measurement of a voltage may be referred toherein. In such cases, it is the magnitude of the voltage that isgreater than (or less than) the other voltage, or that is measured.

Turning now to FIG. 2, a flowchart is shown illustrating one embodimentof testing integrated circuit 10 shown in FIG. 1 prior to packaging theintegrated circuit. The blocks shown in FIG. 2 may be performed on atest machine (e.g. a wafer tester) during the manufacture of theintegrated circuit 10.

The test may begin by testing for a rough characterization of theintegrated circuit 10 (block 30), using various measurements to estimatewhether the integrated circuit is relatively fast, relatively slow, etc.For example, in one embodiment the rough characterization may includetesting the current into the integrated circuit 10 while the integratedcircuit 10 is quiescent (often referred to as “I_(ddq)” testing). HigherI_(ddq) measurements may indicate higher leakage (e.g. a “faster”process). Lower I_(ddq) measurements may indicate lower leakage (e.g. a“slower” process). The I_(ddq) testing may be performed, e.g., with thesupply voltage set to the maximum value permissible for the integratedcircuit 10. From the rough characterization (and from the previousresults of testing instances of the integrated circuit 10), a relativelysmall set of test supply voltages may be selected. That is, based onsupply voltages that provide reliable operation on previous parts havingsimilar rough characterizations, a small set of test voltages may beselected (block 32). For example, a set of three test voltages may beselected, in one embodiment. FIG. 12 is a graphical representation ofthe distribution of parts from fast process (left side of FIG. 12) toslow process (right side of FIG. 12). The test voltages for one examplemay be V₁, V₂, and V₃ as illustrated in FIG. 12.

The test machine may power up the integrated circuit 10 (e.g. with thehighest of the test voltages) and may set the test frequency (one of thefrequencies at which operation of the integrated circuit 10 issupported—block 34). The test machine may run one or more test patternson the integrated circuit 10 for each of the set of test voltages (block36) and may select the lowest test voltage for which all of the testpatterns pass (i.e. the correct result is achieved for eachpattern—block 38). If there are more test frequencies (e.g. moresupported operating frequencies for the integrated circuit 10 that havenot yet been tested—decision block 40, “yes” leg), the next frequencymay be selected and tested (blocks 34, 36, and 38). The set of testvoltages may include different voltages to be tested for each supportedoperating frequency, or may be selected so that at least one passingsupply voltage is expected for each supported operating frequency. Oncethe test frequencies have been exhausted (decision block 40, “no” leg),the test machine may write the frequencies and voltage magnitudes to theF/V table 22 (block 42). For example, fuses may be blown to representthe supported frequencies and corresponding supply voltage magnitudes.

Since the number of test voltages is limited, the test process may notidentify the lowest supply voltage that would result in correctoperation of the particular instance of the integrated circuit 10.However, time on the test machine may be limited, which can be importantin general and especially if the integrated circuit 10 is expected to bemanufactured in high volumes.

Turning next to FIG. 3, a block diagram illustrating operation of oneembodiment of the self calibration unit 16 (and the local power manager18) to perform a self calibration. The self calibration may be performedat various times, as discussed in more detail below. While the blocksare shown in a particular order for ease of understanding, other ordersmay be used. Blocks may be performed in parallel in combinatorial logiccircuitry within the self calibration unit 16 and/or the local powermanager 18. Blocks, combinations of blocks, and/or the flowchart as awhole may be pipelined over multiple clock cycles.

The self calibration unit 16 may communicate to the local power manager18 to indicate that the self calibration process is being performed.Alternatively, the local power manager 18 may initiate the selfcalibration process and may thus be aware that self calibration is inprocess. In either case, the local power manager 18 may request thesupply voltage magnitude that is provided in the F/V table 22 for thetest frequency (block 50). Each operating frequency that is supported bythe integrated circuit 10 may be a test frequency, e.g. beginning withthe lowest frequency. The local power manager 18 may set the testfrequency (block 52), and may wait for the integrated circuit tostabilize on the test frequency (e.g. PLL lock time and/or the settlingtime for the voltage from the PMU/power supply 12). The self calibrationunit 16 may run the self calibration test (block 54) and determine ifthe logic circuit 14 produces the correct result (a pass) or not (afail) (decision block 56). If the test passes (decision block 56, “yes”leg), the self calibration unit 16 may inform the local power manager18, which may request the next lower supply voltage (block 58) and thetest may be performed again (blocks 54 and 56). The test may be iterateduntil a fail result is detected for the test (blocks 54, 56, and 58).Once a fail is detected (decision block 56, “no” leg), the local powermanager 18 may record the magnitude of the lowest passing supply voltagein the self calibration table 20 (block 60). In some embodiments, amargin may be added to the lowest passing supply voltage to arrive atthe voltage magnitude to be recorded in the self calibration table.Alternatively, the margin may be added when requesting the supplyvoltage. If there are more test frequencies to be self calibrated(decision block 62, “yes” leg), the self calibration process returns toblock 50 for the next frequency. Otherwise (decision block 62, “no”leg), the self calibration process ends.

Turning now to FIG. 4, a flowchart is shown illustrating operation ofone embodiment of the local power manager 18 in response to a request tochange the operating frequency (e.g. from software executing on theintegrated circuit 10 or elsewhere in the system that includes theintegrated circuit 10). While the blocks are shown in a particular orderfor ease of understanding, other orders may be used. Blocks may beperformed in parallel in combinatorial logic circuitry within the localpower manager 18. Blocks, combinations of blocks, and/or the flowchartas a whole may be pipelined over multiple clock cycles.

The local power manager 18 may check the self calibration table 20 foran entry corresponding to the new (requested) operating frequency(decision block 70). If an entry is found (decision block 70, “yes”leg), the local power manager 18 may request the supply voltage of themagnitude indicated in the self calibration table 20 (block 72). Thelocal power manager 18 may set the new operating frequency (block 74),and may optionally wait for the clocking circuitry to lock to the newoperating frequency, depending on the implementation (block 76). On theother hand, if there is no entry in the self calibration table 20 forthe requested frequency (decision block 70, “no” leg), the local powermanager 18 may determine if self calibration should be run for therequested frequency (decision block 78). For example, the flowchart ofFIG. 3 may be performed with the requested frequency as the only testfrequency. Factors that may affect whether or not to perform selfcalibration during the frequency change may include the current workloadof the logic circuit 14, the overall environment in the system (e.g.temperature, battery life remaining, etc.). For example, if the logiccircuit 14 includes multiple processor cores and one of the cores isidle, self calibration may be run on the idle processor core. If thesystem is operating on battery power and the battery life remaining islow, running the self calibration may drain more battery power thandesired.

If the local power manager 18 determines that self calibration is to beexecuted (decision block 78, “yes” leg), the local power manager 18 mayinvoke the self calibration unit 16 to perform the self calibration(block 80). The local power manager 18 may then request the supplyvoltage indicated in the self calibration table 20 (after completion ofthe self calibration—block 72), set the new operating frequency (block74), and optionally wait for lock (block 76).

If the local power manager 18 determines that self calibration is not tobe executed (decision block 78, “no” leg), the local power manager 18may read the F/V table 22 to obtain the supply voltage magnitude, andmay request that supply voltage magnitude (block 82). The local powermanager 18 may set the new frequency, and optionally wait for lock(blocks 74 and 76).

In the embodiment of FIG. 4, self calibration may be performed inresponse to a requested operating frequency for which an entry in theself calibration table 20 is not found. In addition to or instead ofthis operation, self calibration may be invoked at one or more otherpoints in time (e.g. as shown in the flowchart of FIG. 5, for oneembodiment). The flowchart shown in FIG. 5 may be implemented inhardware, in software, and/or a combination thereof

If the system that includes the integrated circuit 10 is being bootedfor the first time (e.g. by the customer who purchased thesystem—decision block 90, “yes” leg), the integrated circuit 10 mayexecute self calibration (block 92). Generally, booting a system mayrefer to powering the system on and initializing the system to beginoperation. The determination that the boot is the first boot of thesystem may be made in a variety of fashions. For example, there may be aflag stored in a non-volatile memory in the system that may indicatewhether or not this system is being booted for the first time. The flagmay be checked by boot code in the system, and the flag's state may bechanged at the end of the boot code if the boot is the first boot, sothat subsequent boots may not be detected as the first boot. Forexample, the flag may be a bit that is clear initially, and is set afterthe first boot (or vice versa). In some embodiments, a full system reset(e.g. a hard reset initiated by a user activating one or more inputs tothe device) may clear the “first boot” flag and cause self calibrationto occur on the next boot. Such operation may, in some embodiments,improve functionality of the device. For example, if the user initiatesa hard reset because the device is “frozen” or otherwise malfunctioning,the self calibration may alleviate the error if the error is due tomalfunction in the integrated circuit 10 (e.g. due to a self calibrationsupply voltage magnitude that is too low). Additionally, if the deviceis connected to a network (e.g. the Internet), an updated calibrationprogram or procedure may be downloaded to the device automatically fromthe device manufacturer. The self calibration may be executed inresponse to the update.

Alternatively or in addition, the system may determine that a givenworkload is being executed for the first time (decision block 94, “yes”leg), and may execute the self calibration in response (block 92).Determining that a given workload is being executed for the first timemay be implemented in a variety of fashions (e.g. a flag for eachworkload in non-volatile storage, similar to the discussion aboveregarding first boot). Detecting different workloads may be used, e.g.,in a system in which the workloads vary significantly. For example, thesystem may be a mobile device that may function as a mobile phone, amusic player, a web browser, and may perform various other computingtasks. The workloads may differ substantially, and may require differentamounts of performance from the integrated circuit 10. Accordingly, selfcalibrating each workload may results in additional power savings (e.g.a lighter load may result in a lower operating temperature, which maypermit a lower supply voltage magnitude than heavier workloads wouldpermit).

In yet another alternative or addition, the system may determine that ithas aged by a certain amount (decision block 96, “yes” leg), and mayexecute self calibration in response (block 92). Performing selfcalibration in response to aging of the integrated circuit 10 (and/orthe device that includes the integrated circuit 10) may adjust therequested supply voltage magnitudes for the integrated circuit 10 tocompensate for chip process aging effects or other aging effects. Inthis fashion, margin need not be added to the requested supply voltagemagnitude to account for aging effects (since it is already accountedfor by recalibrating as the integrated circuit 10 ages). Age of theintegrated circuit 10 may be measured in a variety of fashions. Forexample age may be measured from the date of the first boot, based oncalendar time. Age may be measured in terms of time of operation fromthe first boot. Age may be measured in terms of time, or in terms ofclock cycles, as desired. Age may also be measured relative to date ofmanufacture, in other embodiments. In either case, the self calibrationmay be performed at multiple different ages (e.g. once every 6 months,once a year, etc.). In still other cases, self calibration may beperformed dynamically while the system is in operation, which may helpcompensate for temperature effects. Any desired set of self calibrationinvocations may be implemented in various embodiments.

Turning now to FIG. 6, a block diagram of another embodiment of theintegrated circuit 10 and the PMU/power supply 12 is shown. Similar tothe embodiment of FIG. 1, the embodiment of the integrated circuit 10 inFIG. 6 includes the logic circuit 14 and the local power manager 18. Insome embodiments, the self calibration unit 16 and the self calibrationtable 20 may be included, but other embodiments may not include thesefeatures. In the embodiment of FIG. 6, the F/V table 22 from FIG. 1 isreplaced by and F/V/N table 102 coupled to the local power manager 18.The F/V/N table 102 may include entries storing the frequency andcorresponding supply voltage magnitudes, similar to the F/V table 22. Inaddition, the entries may store a delay measurement (N) described inmore detail below. As further illustrated in the embodiment of FIG. 6,the integrated circuit 10 may include a measurement unit 100 and logicgates 104A-104H coupled in series. An input to logic gate(s) 104A iscoupled to the measurement unit 100, and an output of logic gate(s) 104His coupled to the measurement unit 100 as well. Additionally, a flop 106stores an expected delay measurement (N) and a flop 108 stores a countervalue (Ctr). Both flops 106 and 108 are coupled to the measurement unit100. The flops 106 and 108 may be any clocked storage devices, in otherembodiments.

The measurement unit 100 may be configured to measure a propagationdelay of a logical transition through the series connection of gates104A-104H. The gates 104A-104H may have the same design as the variouslogic gates in the logic circuit 14. Accordingly, the propagation delaythrough the gates 104A-104H should be proportional to the logic gates inthe logic circuit 14. By measuring the propagation delay and comparingit to a predetermined delay, the effects of various factors on theoperation of the logic circuit 14 may be accounted for. For example, theeffect of operating temperature, aging, etc. may be detected bymeasuring the propagation delay and comparing it to the predeterminedamount.

The propagation delay may be measured in any desired units (e.g.nanoseconds, clock cycles, etc.). In one embodiment, the propagationdelay is measured in terms of clock cycles at the current operatingfrequency of the clock supplied to the logic circuit 14. Accordingly,the measurement unit 100 may launch a logical transition (e.g. a zero toone or a one to zero transition) into the input of the series connectionof gates 104A-104H (i.e. the input of gates 104A in FIG. 6) and maycount clock cycles until the corresponding transition is detected at theoutput of the series connection (i.e. the output of gates 104H in FIG.6). In one embodiment, a pulse comprising two logical transitions (e.g.zero to one and back to zero again) may be transmitted. The counter Ctrin flop 108 may be cleared when the logical transition is launched andmay be incremented each clock cycle until the corresponding transitionis detected. The flop 106 may store the predetermined number of clockcycles (N) that are expected to occur if the supply voltage is providinga delay that supports the current operating frequency. If the measurednumber of clock cycles is higher than the predetermined number N, thesupply voltage may be increased to lower the delay. If the measurednumber of clock cycles is lower than the predetermined number N, thesupply voltage may be decreased to increase the delay (and consume lesspower).

The number of gates in the series connection may be significantly largerthan the number of gate delays that may evaluate within a clock cycle ofthe clock supplied to the logic circuit 14. For example, the number ofgates in series may be approximately 100 times the number of gate delaysin a clock cycle. Thus, if 14 gate delays are available in the clockcycle, about 1400 gates may be in series in the gates 104A-104H. Using alarge number of gates may improve the matching of the measured delay tothe circuit delay actually occurring in the logic circuit 14.Additionally, because the present embodiment counts the delay in termsof clock cycles, the large number of gates may reduce the measurementerror that occurs due to the clock cycle granularity. For example, at100 times the number of gate delays in a clock cycle, an error of onefull clock cycle in the delay (the maximum possible error) is only 1% ofthe measurement. While 100 is used in this embodiment, other embodimentsmay use larger or smaller numbers (e.g. 200, 500, 100, 50, etc.).

The predetermined number N may be measured during manufacturing test ofthe integrated circuit. The predetermined number N is generally expectedto be close the multiple of the number of gate delays used to create theseries connection of gates (e.g. 100 in the above example), but may varysomewhat from the number. In one embodiment, the predetermined number Nmay be stored in the F/V/N table 102 along with the static supplyvoltage magnitude for a given operating frequency. There may be one Nstored in the table, or there may be one N for each operating frequency(in the entry corresponding to that operating frequency) in variousembodiments.

The gates 104A-104H may be physically distributed over the area of theintegrated circuit 10 that is occupied by the logic circuit 14.Accordingly, variations in process characteristics and/or operatingtemperature that may occur over the surface area of the integratedcircuit chip may be represented in the propagation delay. That is, eachset of one or more gates 104A-140H may be affected by the operatingtemperature and/or process characteristics that are local to thephysical area in which those gates 104A-104H are located. In oneembodiment, the gates 104A-104H may be selected from the “spare gates”that are typically included throughout an integrated circuit 10 in orderto permit repairing logic errors in the logic circuit 14 by changing thewiring layers of the integrated circuit. That is, the spare gates arenot initially wired into the logic circuit 14, and are not used. Iferrors in the logic are detected, the spare gates can be wired into thelogic circuit 14 to generate the correct logic function. A variety ofdifferent logic gates may be included in the spare gates to increase theprobability that the correct logic function can be generated.Accordingly, unused spare gates may have the variety, and may be wiredtogether to create a series connection of gates 104A-104H that may scalesimilar to the logic circuit 14. By implementing the gates 104A-104H outof the spare gates, the gates 104A-104H may not add to the semiconductorarea consumed by the integrated circuit 10.

Additionally, the use of a series connection of logic gates 104A-104H tosense delay is primarily a digital circuit. Thus, the use of the circuitmay be relatively simple and low power, as compared to analog circuitry,in some embodiments.

The measurement unit 100 comprises at least the circuitry configured tolaunch the transition and to measure the propagation delay. In someembodiments, the measurement unit 100 may also include circuitryconfigured to determine when to take the measurement and/or thecircuitry configured to compare the propagation delay to the expectedvalue. Alternatively, the determination may be made in the local powermanager 18 or in software.

Generally, a logic gate comprises circuitry that receives one or moreinputs and is configured to perform a logic function on the inputs toprovide one or more outputs. One or more such gates may be included ineach set of gates 104A-104H. It is noted that, while the gates 104A-104Happear near the periphery of the logic circuit 14 in FIG. 6 forconvenience in the drawing, the gates may generally be interspersedthrough the logic circuit 14 area, as mentioned above.

While the above discussion of FIG. 6 has referred to requesting a supplyvoltage magnitude, and the PMU/power supply 12 supplying a voltage ofthe requested magnitude, the discussion is not meant to imply that thereis only one requested/supplied voltage. There may be multiple supplyvoltages requested and supplied at any given point in time. For example,there may be separate supply voltages for combinatorial logic circuitryand for memory circuitry in the logic circuit 14. There may be multiplevoltage domains within the integrated circuit 10 that may be powered upand down separately, and may have separate measurement units and serialchains of logic gates. Each such domain may include a separate request.The local power manager 18 may be powered separate from the logiccircuit 14. Any set of one or more supply voltages may be requested andsupplied. Furthermore, in some embodiments, more than one chain of gatesmay be implemented within a voltage domain to model different types ofdelays. For example, logic gate delays and register file delays may bemodeled separately.

Turning now to FIG. 7, a flowchart is shown illustrating one embodimentof testing integrated circuit 10 as shown in FIG. 6 prior to packagingthe integrated circuit. The blocks shown in FIG. 7 may be performed on atest machine (e.g. a wafer tester) during the manufacture of theintegrated circuit 10.

Similar to the embodiment of FIG. 2, the test may begin by testing for arough characterization of the integrated circuit 10 (block 30), such asI_(ddq) testing, and a set of test voltages may be selected (block 32).The tester may set a first test frequency (block 34), and may run a testpattern for each voltage in set of test voltages as the supply voltage(block 36). In this embodiment, the test may include activating themeasurement unit 100. For each test voltage, the tester may read thenumber of clock cycles of delay “N” as measured by the measurement unit100 during the test (that is, the value in the counter flop 108). Thetester may record the measured N for each test voltage (block 110). Thetester may then select the minimum supply voltage magnitude for whichthe test passes, as well as the corresponding “N” (block 112).

If there are more test frequencies (e.g. more supported operatingfrequencies for the integrated circuit 10 that have not yet beentested—decision block 40, “yes” leg), the next frequency may be selectedand tested (blocks 34, 36, 110, and 112). The set of test voltages mayinclude different voltages to be tested for each supported operatingfrequency, or may be selected so that at least one passing supplyvoltage is expected for each supported operating frequency. Once thetest frequencies have been exhausted (decision block 40, “no” leg), thetest machine may write the frequencies, voltage magnitudes, and valuesof “N” to the F/V/N table 102 (block 114).

In some embodiments, a margin may be added to the measured “N” to bewritten to the table. Alternatively, the margin may be added to “N” readfrom the table to write “N” to the flop 106. Similarly, a margin may beadded to the supply voltage magnitude written to the table or the marginmay be added by the local power manager 18 after reading the magnitudefrom the table.

Turning now to FIG. 8, a flowchart is shown illustrating operation ofone embodiment of the integrated circuit 10 as shown in FIG. 6 (and moreparticularly the measurement unit 100 and the local power manager 18) todetermine if the supply voltage magnitude is to be adjusted. Theoperation of FIG. 8 may be performed periodically while the integratedcircuit 10 is in operation (e.g. approximately once every 10microseconds to 1 millisecond, in one embodiment, depending on thethermal inertia of the system). The operation of FIG. 8 may be performedafter a change in workload (e.g. from a phone to a music player or amobile internet access device). The operation of FIG. 8 may be performedas part of changing the frequency of operation, as well. The blocks areshown in a particular order for ease of understanding, but other ordersmay be used. Blocks may be performed in parallel by combinatorial logicwithin the measurement unit 100/local power manager 18. Blocks,combinations of blocks, and/or the flowchart as a whole may be pipelinedover multiple clock cycles.

The local power manager 18 may activate the measurement unit 100, whichmay sense the current propagation delay (“N”) in the series connectionof gates 104A-104H (block 120). In some embodiments, the local powermanager 18 and/or the measurement unit 100 may filter the results (block122). Specifically, for example, the filtering may include detectingoscillation of the value of N between consecutive measurements. Theoscillation may occur because the propagation delay is close an integernumber of clock cycles (and thus is sometimes captured in M clock cyclesand other times is captured in M+1 clock cycles). The oscillation mayalso occur because the requested supply voltage is being increased anddecreased in an oscillatory manner.

If the measurement unit 100 detects that the measured “N” is greaterthan the “N” from the F/V/N table 102 (decision block 124, “yes” leg),the local power manager 18 may increase the requested supply voltagemagnitude sent to the PMU/power supply 12 (block 126). For example, thenext higher supply voltage magnitude may be requested. If themeasurement unit 100 detects that the measured “N” is less than the “N”from the F/V/N table 102 (decision block 128, “yes” leg), the localpower manager 18 may decrease the requested supply voltage magnitudesent to the PMU/power supply 12 (block 130). The operation of FIG. 8 maybe repeated until the requested supply voltage magnitude settles, or maybe repeated at the next measurement time, as desired.

Turning now to FIG. 9, a flowchart is shown illustrating operation ofone embodiment of the integrated circuit 10 (and more particularly thelocal power manager 18 and the measurement unit 100) in response to arequest to change frequencies in the integrated circuit 10. The blocksare shown in a particular order for ease of understanding, but otherorders may be used. Blocks may be performed in parallel by combinatoriallogic within the measurement unit 100/local power manager 18. Blocks,combinations of blocks, and/or the flowchart as a whole may be pipelinedover multiple clock cycles.

If the request to change frequencies is an increase from the currentoperating frequency (decision block 140, “yes” leg), the value of N inthe flop 106 may be scaled by the ratio of the new (requested) frequencyand the old (current frequency) (block 142). For example, if the currentfrequency is 1 GHz and the new frequency is 1.5 GHz, the value of N maybe scaled by 1.5. The local power manager 18 and the measurement unit100 may iterate the adjust supply voltage process of FIG. 8 until themeasured N from the measurement unit 100 matches the scaled N (block144). In some embodiments, a margin may be added to the scaled N toensure that the increased supply voltage magnitude is sufficient tosupport the newly changed frequency. Once the scaled N is met, the localpower manager 18 may set the new frequency (block 146) and may wait forthe clock generation circuit to lock to the new frequency (block 148). Anew N may be read from the F/V/N table 102 and may be written to theflop 106 (block 150).

If the request to change frequencies is a decrease from the currentoperating frequency (decision block 140, “no” leg), the local powermanager 18 may set the new frequency (block 146) without scaling N andadjusting the supply voltage (blocks 142 and 144). Since the supplyvoltage is already high enough to support the higher current frequency,the integrated circuit 10 will operate at the new frequency correctly.Subsequent periodic measurements and adjustments (e.g. FIG. 8) may lowerthe voltage. In other embodiments, N may be scaled and the voltage maybe adjusted (blocks 142 and 144) for the lower new frequency as well, inwhich case the adjustments to the supply voltage will be reductions.

In some embodiments, the process of adjusting the supply voltagemagnitude (block 144) may begin with reading the supply voltagemagnitude from the F/V/N table 102 (or the self calibration table 22)for the new frequency and initializing the process by requesting thesupply voltage magnitude from the table.

Turning next to FIG. 10, a flowchart is shown illustrating anotherembodiment of testing integrated circuit 10 as shown in FIG. 6 prior topackaging the integrated circuit. The blocks shown in FIG. 10 may beperformed on a test machine (e.g. a wafer tester) during the manufactureof the integrated circuit 10.

Similar to the embodiment of FIG. 2, the test may begin by testing for arough characterization of the integrated circuit 10 (block 30), such asI_(ddq) testing. Additionally, the measurement unit 100 may be activatedwith the supply voltage set to its maximum possible value (per thespecification of integrated circuit 10) (block 160). The measurement ofN at the maximum possible value may be an indication of the “speed” ofthe integrated circuit 10, and may be used to select a set of testvoltages (block 162). In this fashion, the selected test voltages may becloser to the optimal voltage for a given frequency, which may permitfine grain voltage testing in a short amount of test time and resultingsupply voltage magnitude that is close to the optimum value for theintegrated circuit 10. Accordingly, the integrated circuit 10 mayconsume less power at a given frequency when the supply voltage is setto the voltage in the table (as compared to less optimal teststrategies). Additionally, a relatively small set of voltages may stillbe used, reducing test time. For example, FIG. 13 is a graphicalrepresentation of the distribution of parts from fast process (left sideof FIG. 13) to slow process (right side of FIG. 13). Using the maximumvoltage (dotted line farthest to the right), N may be measured. Based onthe measured N, a small set of test voltages near the expected operatingpoint may be selected and the integrated circuit 10 may be tested atthese voltages (brace at the bottom of FIG. 13).

Subsequently, similar to the embodiment of FIG. 7, set the first testfrequency (block 34), and may run a test pattern for each voltage in setof test voltages as the supply voltage (block 36). The test may includeactivating the measurement unit 100. For each test voltage, the testermay read the number of clock cycles of delay “N” as measured by themeasurement unit 100 during the test (that is, the value in the counterflop 108). The tester may record the measured N for each test voltage(block 110). The tester may then select the minimum supply voltagemagnitude for which the test passes, as well as the corresponding “N”(block 112).

If there are more test frequencies (e.g. more supported operatingfrequencies for the integrated circuit 10 that have not yet beentested—decision block 40, “yes” leg), the next frequency may be selectedand tested (blocks 34, 36, 110, and 112). The set of test voltages mayinclude different voltages to be tested for each supported operatingfrequency, or may be selected so that at least one passing supplyvoltage is expected for each supported operating frequency. Once thetest frequencies have been exhausted (decision block 40, “no” leg), thetest machine may write the frequencies, voltage magnitudes, and valuesof “N” to the F/V/N table 102 (block 114).

In some embodiments, a margin may be added to the measured “N” to bewritten to the table. Alternatively, the margin may be added to “N” readfrom the table to write “N” to the flop 106. Similarly, a margin may beadded to the supply voltage magnitude written to the table or the marginmay be added by the local power manager 18 after reading the magnitudefrom the table.

Turning next to FIG. 11, a flowchart is shown illustrating operation ofone embodiment of the measurement unit 100 to perform a measurement. Themeasurement unit 100 may perform the operation illustrated in FIG. 11 inresponse to the local power manager 18 initiating a measurement, forexample. The blocks are shown in a particular order for ease ofunderstanding, but other orders may be used. Blocks may be performed inparallel by combinatorial logic within the measurement unit 100. Blocks,combinations of blocks, and/or the flowchart as a whole may be pipelinedover multiple clock cycles.

The measurement unit 100 may clear the counter in the flop 108(reference numeral 170), and may launch a logical transition into theseries connection (or “chain”) of gates 104A-104H (block 172). If themeasurement unit 100 has not yet detected the corresponding logicaltransition at the output of the chain (decision block 174, “no” leg),the measurement unit 100 may increment the counter (block 176) and waitfor the next clock cycle to detect the transition again (block 178). Ifthe measurement unit 100 has detected the corresponding transition(decision block 174, “yes” leg), the measurement unit 100 may comparethe counter to N in the flop 106 and report the results to the localpower manager 18 (block 180).

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

1. A method comprising: performing a characterization of an integratedcircuit at a first supply voltage magnitude; responsive to a result ofthe characterization, selecting a subset of a plurality of test supplyvoltage magnitudes, wherein the subset excludes at least one of theplurality of test supply voltage magnitudes; testing the integratedcircuit at each of the test supply voltage magnitudes in the subset,wherein the testing is performed at a first clock frequency of theintegrated circuit; determining a minimum supply voltage magnitude ofthe subset at which the testing is successful; and storing an indicationof the minimum supply voltage magnitude in the integrated circuit foruse during operation of the integrated circuit at the first clockfrequency, wherein the integrated circuit includes a self calibrationunit configured to reduce a supply voltage magnitude below the minimumsupply voltage at the first clock frequency responsive to selfcalibration testing during use.
 2. The method as recited in claim 1wherein performing the characterization comprises measuring a current inthe integrated circuit while the integrated circuit is quiescent.
 3. Themethod as recited in claim 2 wherein the first supply voltage magnitudeis a maximum supported supply voltage magnitude for the integratedcircuit.
 4. The method as recited in claim 1 further comprising:powering the integrated circuit with a supply voltage having the minimumsupply voltage magnitude; performing a plurality of iterations of theself calibration by the self calibration unit, each of the plurality ofiterations at successively lower supply voltage magnitudes than theminimum supply voltage magnitude; responsive to the plurality ofiterations of self calibration, selecting a calibrated supply voltagemagnitude that is lower than the minimum supply voltage magnitude; andoperating the integrated circuit at the calibrated supply voltage andthe first clock frequency.
 5. The method as recited in claim 4 furthercomprising: storing an indication of the calibrated supply voltagemagnitude in the integrated circuit; and using the calibrated supplyvoltage in a subsequent powering of the integrated circuit, whereby theself calibration unit is not active in the subsequent powering of theintegrated circuit.
 6. The method as recited in claim 5 wherein storingthe indication of the calibrated supply voltage magnitude compriseswriting the indication to a volatile memory in the integrated circuit.7. The method as recited in claim 1 wherein storing the indication ofthe minimum supply voltage magnitude comprises writing the indication toa table.
 8. The method as recited in claim 7 wherein writing theindication comprises selectively blowing fuses that form the table. 9.An integrated circuit comprising: a logic circuit; a local power managercoupled to the logic circuit and configured to transmit an indication ofa requested supply voltage magnitude to an external power supply; atable coupled to the local power manager and configured to store a listof operating frequencies and supply voltage magnitudes, wherein thesupply voltage magnitude for each operating frequency is determinedduring test of the integrated circuit based on a characterization and atest of a subset of a plurality of test supply voltage magnitudes; and aself calibration unit within the integrated circuit and configured toexecute a test on the logic circuit; wherein the local power manager isconfigured to power the integrated circuit at a selected operatingfrequency using a corresponding power supply voltage magnitude from thetable, and wherein the self calibration unit is configured to iteratethe test at respectively lower requested supply voltage magnitudescompared to the corresponding power supply voltage magnitude, andwherein the local power manager is configured to use a lowest requestedsupply voltage magnitude at which the test passes to determine a firstrequested supply voltage magnitude for operation of the integratedcircuit at the selected operating frequency.
 10. The integrated circuitas recited in claim 9 further comprising a self calibration tableconfigured to store a list of operating frequencies and correspondingsecond supply voltage magnitudes determined by the self-calibrationunit, wherein the local power manager is configured to request the firstrequested supply voltage magnitude for the selected operating frequencyresponsive to the self calibration table.
 11. The integrated circuit asrecited in claim 10 wherein the self calibration unit is configured towrite the first requested supply voltage magnitude as the correspondingsecond supply voltage magnitude to the self calibration table for theselected operating frequency responsive to determining the lowestrequested supply voltage magnitude.
 12. The integrated circuit asrecited in claim 10 wherein the local power manager is configured toactivate the self calibration unit responsive to detecting that the selfcalibration table does not have a valid entry for the selected operatingfrequency.
 13. The integrated circuit as recited in claim 10 wherein thelocal power manager is configured not to activate the self calibrationunit responsive to detecting that the self calibration table does have avalid entry for the selected operating frequency.
 14. A methodcomprising: detecting a request in an integrated circuit to change froma first operating frequency to a second operating frequency; reading anentry in a self calibration table, wherein the entry is associated withthe second operating frequency; and responsive to detecting that theentry is not storing a valid supply voltage magnitude, activating a selfcalibration unit to determine a supply voltage magnitude for the secondoperating frequency.
 15. The method as recited in claim 14 furthercomprising: setting a requested supply voltage magnitude to a magnituderead from a second table responsive to detecting that the entry in theself calibration table is not storing the valid supply voltagemagnitude; iterating a test in the self calibration unit, wherein therequested supply voltage magnitude is reduced for each iteration until atest failure results; determining a lowest requested supply voltagemagnitude at which the test passes; and writing a corresponding supplyvoltage magnitude to the entry.
 16. The method as recited in claim 15wherein the corresponding supply voltage magnitude is derived from thelowest requested supply voltage magnitude by including margin above thelowest requested supply voltage magnitude.
 17. The method as recited inclaim 14 further comprising: detecting a second request in an integratedcircuit to change from the second operating frequency to a thirdoperating frequency; reading a second entry in the self calibrationtable, wherein the second entry is associated with the third operatingfrequency; and responsive to detecting that the second entry is storinga valid supply voltage magnitude, not activating the self calibrationunit.
 18. The method as recited in claim 17 further comprising:requesting the valid supply voltage magnitude from the second entry; andswitching to the third operating frequency.
 19. An integrated circuitcomprising: a logic circuit; a local power manager coupled to the logiccircuit and configured to transmit an indication of a requested supplyvoltage magnitude to an external power supply; a self calibration unitwithin the integrated circuit and configured to execute a test on thelogic circuit; a self calibration table coupled to the local powermanager and configured to store a list of operating frequencies andsupply voltage magnitudes, wherein the supply voltage magnitude for eachoperating frequency is determined by the self calibration unititeratively executing the test at lower requested supply voltagemagnitudes during use; and wherein the local power manager is configuredto detect a request for a change from a first operating frequency to asecond operating frequency, and wherein the local power manager isconfigured to selectively activate the self calibration unit responsiveto whether or not a valid supply voltage magnitude is stored in the selfcalibration table for the second operating frequency.
 20. The integratedcircuit as recited in claim 15 wherein the self calibration unit, whenactivated, is configured to: set the requested supply voltage magnitudeto a magnitude read from a second table responsive to detecting that theentry in the self calibration table is not storing the valid supplyvoltage magnitude; iterate a test, wherein the requested supply voltagemagnitude is reduced for each iteration until a test failure results;determine a lowest requested supply voltage magnitude at which the testpasses; and write a corresponding supply voltage magnitude to the entry.